This invention is in the field of digital micromirror devices. This invention relates generally to micromirror devices employing polymeric structural layers and high reflectivity dielectric multilayers and methods for making and using micromirror devices.
In the fabrication of electrical, opto-electronic, micro system and display products, microelectronic fabrication methods such as deposition, photolithography and etching processes are used to make millions of microstructures or micro devices on a substrate. The substrate for micro fabrication can be silicon wafers, compound semiconductor wafers, glass, polymers, printed circuit boards (PCB), multi chip modules, etc. The size of the substrate ranges from a few square inches in integrated circuits up to a few square meters in displays. On these substrates, microstructures and devices such as transistors, capacitors, resistors, interconnects or any kind of electronic components are fabricated by microelectronic fabrication methods. Pattern sizes that can be fabricated range from submicron features up to a few microns, depending on their purpose.
In the fabrication of microstructures for applications mentioned above, patterning technologies and etching methods are required to make active or passive functional devices. Photolithography technology is commonly used to make patterned materials on a substrate. In photolithography methods, the material is coated or deposited with photoresist (PR) by various kinds of methods. Then, the PR is baked and stabilized for the next process. In the subsequent process, light is illuminated through a patterned mask and then the polymer bonds of the photosensitive chemical component of the PR in the illuminated area are, depending on the tone of the PR, dissociated or cross-linked due to illumination energy. Following this process, again depending on the tone of PR, the illuminated or non-illuminated PR is dissolved by developer and the underlying materials are opened. PR protects the material from being etched in the wet etch and dry etch processes. The materials exposed to the etch environment without a PR covering are etched by dry etching or wet chemical etching. For the last process step, the remaining PR on the material is removed by a PR stripper, resulting in patterned materials on the substrate.
For the lithography methods described above, three kinds of lithography methods are currently used: contact printing methods, projection imaging methods and focused beam laser direct writing methods. In contact printing methods, the mask containing the master pattern mechanically makes contact with the PR coated substrate and then is illuminated by a light source. The chemical property of PR changes due to illumination. There are, however, serious disadvantages which make contact printing methods difficult to use and discouraging industry adoption. Due to the mechanical contact between the mask and PR, the mask could be contaminated by PR where some polymer residue is transferred onto the mask. This defect on the mask can make undesired patterns on future substrates by blocking the illumination.
Projection imaging methods are commonly used for fabrication of integrated circuits (ICs), displays and other microelectronic products. A projection imaging system generally comprises three parts: a light source, which generates light with a desired wavelength, energy and uniformity; optical components which transmit the light from the source to a substrate; and a stage which holds the substrate and allows for high-precision movement for pattern alignment. Among these components, optical components generally comprise several lenses for uniformity and alignment to the substrate.
Two projection imaging method are generally used: scan type projection and stepping type projection. In scan type projection systems, the illumination is scanned on the entire substrate area by moving the optical parts or the substrate. An advantage of this method is large-size availability because this technique illuminates large area at one time. This method can be used for displays or large size IC fabrications which require a large-size substrate. However, a primary disadvantage of this method is the price of photolithography systems. For photolithography systems, if the sizes of the lenses in the optical parts become larger, the price of the system becomes much higher than those of other mechanical parts.
In stepping type projection methods, the whole substrate area is divided into several segments and is illuminated segment by segment in order. This method is useful when repetitive patterns are generated on a substrate. For special purpose such as large size display fabrication with an area of several square meters, both scanning and stepping methods are used. A primary disadvantage of the stepping method is its non-uniform illumination on a large size substrate. Due to the mechanical alignment issues in the stepping method, there can be regions where the illumination is duplicated or not illuminated as shown in FIG. 1. The substrate 101 is illuminated by three stepping shots 102, 103 and 104. If there is a misalignment in the second shot 103, the illumination energy can be doubled in the region 105 where the illumination is duplicated. However, between the second shot 103 and third shot 104, there will be a non-illuminated region 106, where the energy of illumination is zero. The patterns on the border area can also be over-developed or under-developed because there is difference of illumination energy. In the case of display fabrication such as TFT-LCD, the non uniform pattern in the substrate can make non-uniform images. This problem is referred to as the stitching problem, and there have been extensive efforts to reduce the problem.
The third patterning method is called the laser ablation method. A laser is illuminated through a patterned mask, and the selected area on the substrate is illuminated by the laser. Due to the high energy density of the laser source, the material on the substrate will undergo chemical reactions, physical reactions or other types of reactions. Essentially, the illuminated area is removed by the laser ablation and only the non-illuminated area will remain, and thus the patterning is accomplished by laser illumination. This method is very simple and economical because it does not need a PR coating system, a developing system, a bake system or a PR stripping system. Thus, the total fabrication time of the whole product also decreases due to the reduced number of fabrication steps, and the resulting reduction in fabrication cost. However, this method is not commonly used for the display industry because the materials that can be ablated by laser are limited. Among the materials that are used in TFT-LCD or PDP process is ITO (Indium Tin Oxide), a type of transparent conductor. This material can be ablated by excimer laser illumination. Also, some types of organic materials can be ablated by a laser.
For the fabrication of patterns on a substrate using methods involving illumination by radiation, a photomask is required for selective illumination on the substrate. FIG. 2 shows how a photomask is utilized in the photolithography process. In lithography equipment such as the stepper or scanner, illumination source 201 generates illumination radiation 202. Optics components 203 and 204 are required to make images on the substrate 205. In the middle of the illumination path, a photomask 206 is placed to differentiate areas on the substrate 205. The photomask consists of transparent and opaque areas, so that the illumination through the openings of the photomask 206 reaches the substrate 205 and other illumination is blocked by photomask 206. A photosensitive layer, usually photoresist, is coated on the substrate 205 and the illumination causes photochemical effects on the photoresist.
A photomask typically comprises a base material 301, a mask material 302 and an antireflective material 303, as shown in FIG. 3. For the base material 301, fused silica (quartz), calcium fluoride or glass is commonly used because such material has high transparency for the wavelength of the common illumination source. For the masking material 302, chrome is commonly used but other types of materials such as aluminum can also be used. The antireflective material 303 is used to avoid unwanted images generated by reflected light from substrate.
In an example photomask fabrication method, chrome 302, antireflective layer 303 and e-beam resist are coated on the quartz substrate 301 and patterned by e-beam lithography or other methods. The e-beam resist is removed after making the pattern on the photomask. The dimension, width and length, of chrome 302 and antireflective coating 303 needs to be very precise because the pattern on the photomask is exactly transferred to the substrate. Therefore, a very precise and expensive process is required for the fabrication of photomasks.
One of the most important concerns of photomask production is the price. Once a photomask is fabricated with a certain design, it is permanent and cannot be modified after its fabrication. There are several cost related problems in the industry due to the non-modifiable nature and the expensive cost of the photomask. First, multiple photomasks are typically required for device fabrication. Although the required number of photomasks varies different from device to device, it is common that more than 10 photomasks are required for fabrication of conventional CPU or RAM devices.
Second, in addition to the requirement of photomasks with varying patterns, additional photomasks are further required for mass production to accommodate for revisions and aging. In the production of electronic devices, there are often many revisions on the design of a device for better performance or cost efficiency, and new photomasks are typically purchased for each revision. Also, photomasks generally need replacement after some time period because they tend to degrade when used for a long time. When a photomask is used in mass production, defects caused by particles or scratches are generated on the photomask during usage and a replacement photomask is eventually needed.
Third, considering the use of photomasks in the display industry, the price problem of photomasks becomes more severe because the size of the photomask required is much larger than those used in IC fabrication. For example, the size of the photomask used in a 7th generation TFT-LCD plant is 1220 mm×1400 mm (reference: Nikon FX-71S catalog). The cost of the photomask is not disclosed, but it is well known that the price increases exponentially as the size of photomask increases linearly. It is expected that tens of thousand of dollars are required for such a large size photomask. For these and other reasons, the use of photomasks has become a large burden to the device fabrication industry and flat panel display fabrication industry.